What we know and what we don't know
(c) Stephan Baudy
"...The science of global warming is far
Nigel Lawson, "
An appeal to reason: a cool look at global warming", 2008.
Climate skeptics often claim that although increasing levels of
carbon dioxide may be causing global temperatures to rise, there
uncertainties in climate science - with the implication that it
would be unwise to put in place potentially expensive or disruptive
measures to reduce emissions.
For those not actively working within the field of climate
science it can be difficult to determine just how sure scientists
are of what they are saying.
In order to do so it is important to understand the method by
which science proceeds. Science progresses by testing theories,
examining results, repeating the tests and extending the theories.
It's this process that allows valid results to be confirmed, and
disproven theories to be abandoned. Over time, as a large
supporting body of evidence is built and attempts to disprove a
theory have failed, that theory can be considered sound. Although
we can never be absolutely certain that a theory is true, we can be
confident in it given a large volume of supporting evidence and
numerous failed attempts to disprove it.
(Well, that's the theory. In practice, the process is usually a
So here's a brief article summarising what bits of climate
science we have strong evidence for, and what we are less sure
What we have strong evidence for:
Greenhouse gases, including carbon dioxide, warm the
planet - the theory that gases including carbon dioxide,
methane, nitrous oxide and water vapour retain heat in the
atmosphere was first suggested 150 years ago. Svante
Arrhenius went on to calculate the potential effect of changing
CO2 on climate in 1896. That was followed by over a
century of discussion, testing, and refinement of the theory, a
process which provided a wealth of supporting evidence.
Other researchers initially argued
against the theory, but they were unable to disprove it. The
warming effect of carbon dioxide can in fact be demonstrated
in the laboratory. As Richard Somerville, climate scientist and
distinguished Professor Emeritus at Scripps Institution of
"The greenhouse effect is well
understood. It is as real as gravity."
Atmospheric CO2 is increasing, and has been
since the industrial revolution. This is shown by direct
ice-cores and the
atmosphere. Levels of the greenhouse gases methane and nitrous
oxide have also risen markedly over the same period.
Human activities are causing that rise in CO2.
We know that burning fossil fuels and deforestation releases carbon
dioxide. Scientists have demonstrated that much of the increase in
CO2 in the atmosphere comes as a result of human activities by
studying its isotopic composition. CO2 which comes from burning
fossil fuels has a distinct
manmade signal (explained further
here). The proportion of CO2 in the atmosphere that has this
"human fingerprint" is rising. The rise in other greenhouse gases
such as methane and nitrous oxide can also be
attributed to human activities including agriculture and fossil
The average global temperature is rising at the
surface of the planet (atmospheric and sea surface temperatures).
Since it is near impossible to distinguish temperature trends over
the short term from 'noise' in the data, climate scientists prefer
to consider the longer-term temperature trend. This shows an
increase of around 0.75˚C
since the beginning of the 20th century.
A mass of independent evidence - for
example the rising temperature of the oceans, rising humidity,
rising sea level and melting sea ice - from many different sources
also indicate that the world is warming.
Skeptics frequently cite
solar activity or other natural climate variation to explain
the warming trend. All of these natural elements have been
thoroughly investigated by climate scientists, and have been
ruled out as the dominant cause of warming.
These fundamental principals, added to the fact that there are
no plausible competing theories to explain the observed warming
trend, have convinced
97% of active climate scientists that humans are a significant
contributing factor to global warming. The IPCC concluded in its
report that there is a 90% likelihood that human activities
have led to overall warming since pre-industrial times.
What is still under discussion
What our future emissions will be. Future
emissions are likely to depend on several key factors, including
global population, energy resource availability, technology, and
policy. The IPCC have developed a series of potential greenhouse
emissions scenarios, based on different global development
storylines. These are what are used for the IPCC's climate
projections for the 21st century.
How much temperatures will rise from any given emissions
pathway - There are significant uncertainties in assessing
how much temperatures will rise because of the difficulties
inherent in modelling a system as complex as the climate.
The range of emissions scenarios
project that temperatures will increase by between 1.8˚C and
around 4˚C by the end of this century. We cannot yet tell which of
these emissions scenarios is the most likely to happen - although
it has been suggested that our current emissions could put us on
track for a temperature rise of 4˚C or more.
We cannot know precisely what the effects of temperature
rise will be. Where, when and how severe will the physical
impacts be? How will societies respond? Making these kind of
predictions is particularly difficult when it comes down to the
regional level. When the IPCC assessed the mass of evidence it was
found that the impacts of warming are likely to be predominantly
negative, particularly at the higher projected temperatures. For
example, at a temperature rise of 3 - 4°C above pre-industrial
- Increased water stress for around one sixth of the world's
- 20 - 30% of plant and animal species at increased risk of
- Increased coastal damage from floods and storms
- Oceans acidification and coral mortality
- Diminished net carbon uptake by ocean and land sinks
- Decreased cereal production at low latitudes and in some high
- Increased injury and mortality from heat waves, floods and
droughts - increased burden on health services
Overall, the issue is not "are we having an effect on our
planet?" but "what is the scale and timing of the effect we are
having on our planet?". Uncertainty in one area of climate science
(for example in future projections from climate models) does not
mean other areas (for example the evidence that temperature is
currently rising) are any less robust.
Thanks to PIRC for the
concept and some of the words!